Kerogen has long been an enigma and its meaningful classification an illusive goal.
In the field of geochemical research and petroleum exploration, an area of growing interest is analysis of geological samples to determine the remaining petroleum-generating potential of the rock sample. Work in this field has been directed to methods for removing hydrocarbons from geological samples and analyzing the hydrocarbons, to apparatuses for laboratory and field analysis for geological samples, and to the development of theoretical and practical models for predicting petroleum-exploration-related characteristics for the raw data obtained.
A method known in the art for such analysis involves the thermal extraction of volatile hydrocarbons from a kerogen (insoluble organic portion of sedimentary rock) or rock samples under carefully controlled temperature conditions and the analysis of the hydrocarbons extracted. In this method, a geological sample is heated to about 280.degree. C. at a constant rate of temperature increase. A gas-detecting instrument records signals representative of the quantity of both volatile hydrocarbons and other gases thermally extracted from the sample. The sample is further heated to a higher temperature at a constant rate of temperature increase. A gas-detecting instrument records signals representative of the quantity of hydrocarbons and inorganic gases formed by pyrolyses over some predetermined temperature range. The quantities of hydrocarbons formed can be expressed in units of milligrams hydrocarbon per grams sample. One such thermal method involves the controlled heating of a geological sample and the determination of the amount of hydrocarbons and non-hydrocarbons generated over at least two predetermined temperature ranges.
Other commonly utilized techniques for classifying kerogen lack certain of the virtues of the pyrolytic system. Visual characterization, although currently popular, of necessity, focuses on the larger size, more visible fraction of kerogen rather than the entire mixture, as is the case with pyrolysis. Attempts to relate visual characterization with pyrolytic determinations and chemical analysis have met with very limited success.
A more definitive method popularized by Tissot and now in common usage utilizes ratios of carbon, hydrogen, and oxygen to classify kerogens into three groups. However, when kerogens of diverse composition reach the mature stage, their composition, in terms of the ratios of carbon, hydrogen, and oxygen become more similar. This may cause the elemental ratio data for mature kerogens to lack capacity to support inferences of whether oil and/or gas were generated at earlier stages of maturity.
Therefore, what is needed is a method of classification which will have an advantage in terms of cost and simplicity of sample preparation. Also, what is needed is a method which would give more detail and significant data regarding the basic structure of a kerogen.